Access the light properties (by
right-clicking the light item in the specification tree then selecting
Properties) then access the Indirect Illumination
|Emit Photons is already selected for the
current sample document but note that it is cleared by default.
What are photons?
|Photons are energy particles emitted into the scene
by a light source. Once emitted, these energy particles take
different paths as they bounce from one object to another. A photon
path can evolve in two ways: photons are either absorbed by an
object they hit (through reflection or transmission) and in that
case the path ends, or photons get lost in space without hitting
another object and the path is infinite.
Photon emission can be compared to ray casting, the differences
being that photons are emitted by a light source and not a camera
as for rays and that photons have energies but rays have colors.
How are photons stored?
|As a photon bounces from one object to another,
each rebound on a
diffuse surface (and on a diffuse surface only) is stored in a
three-dimensional data structure called a photon map.
Thus the photon map contains the position and the light energy of
each photon hitting a diffuse surface, these data being used later
to compute indirect illumination.
The only exception in the storing phase is the first reflection or
transmission point which is never taken into account because it is
handled by direct illumination.
|Let`s detail the figure below explaining how
photons are emitted, absorbed and stored through two photon paths:
|The first photon is emitted at the light source,
hits a diffuse wall and is reflected at point A but not stored (it
is the first reflection), and finally hits point B where it is
stored and absorbed.
The second photon is reflected at the diffuse floor (C) then
reflected and stored at the wall (D), transmitted but not stored at
the specular (non-diffuse) sphere surface points E and F, and
finally stored and absorbed on the floor (G).
|A photon is said to be absorbed on a surface when
it is neither reflected nor transmitted. The photon
path ends where it is absorbed.
Absorption can be forced when the trace depth limit is reached.
This limit is defined in the
Shooting Definition dialog box where you indicate the maximum
number of reflections, refractions and rebounds.
|Photon emission stops as soon as a specified number
has been reached or as soon as a maximum number has been emitted.
However, the photon map does not affect the rendering phase since
it is almost entirely independent from the scene complexity.
|The last phase is the rendering phase during which
direct local illumination is computed as usual but, in that case,
indirect global illumination is added by collecting neighbouring
photons from the photon map and adding their contribution.
In the Intensity Factor box,
indicate the multiplication factor of the light intensity associated to
each photon, using either the slider or the spin box.
|By default, direct and indirect lighting have the same
intensity so the factor is equal to 1. A good value for Intensity
Factor should be close to 1, except for a particular purpose (style
effect or especially dark scenes, for instance).
In the Global Illumination Photons
box, define the maximum number of photons to be stored in the photon map.
This value controls the rendering quality of global illumination.
You can enter a value comprised between 10 000 and 10 000 000.
|Be careful when setting this parameter since the
higher the value, the longer the rendering phase and the higher the
quality. On the other hand, too small values may create fuzzy light
Gathering is activated, setting lower number of photons is
sufficient to get a satisfactory result.
OK to validate your lighting parameters then click Create
to define the shooting parameters in the Indirect Illumination
|This tab lets you define global illumination
parameters in order to create color bleeding, i.e. have the colors
reflected from diffuse surfaces.
In the Global Illumination area,
click Active to activate global illumination and activate the
two other boxes.
In the Photons box, indicate for
each pixel the number of photons to be searched in a specified area
(defined in the Maximum radius box) in order to compute the
local intensity of global illumination.
|The default value is 500. The higher the value, the
smoother the illumination but the longer the rendering phase. A
large radius gives less noise but a blurrier result.
Enter the Maximum radius (in
millimeters) to set the limits of the area in which the photons will be
searched to compute globall illumination.
|A null value means that the radius is computed from
the scene extent.
Click Do not use ambient
in the Material area if you do not want the material
ambient coefficient to be taken into account when computing indirect
|Ambient coefficient corresponds to the light
intensity diffused in any direction by the object, even if not lit
by any light source. This parameter affects the whole object,
including the shadowed area.
Note that this option can be selected only when at least one of the
indirect illumination parameters is activated.
OK to validate your parameters then render the shooting using
(you can also right-click the Shooting item in the specification tree
then select Render Single Frame) to check the result.
|In case the
rendered image shows artefacts, you can increase the number of
Note also that increasing the radius value may also help to
increase the color attenuation.